Nicotinate Phosphoribosyltransferase controls aggressiveness, Ca2+ homeostasis, and NAD+-derived Ca2+ mobilizing second messengers in triple-negative breast cancer

Triple-negative breast cancer (TNBC) represents the most aggressive form of breast cancer (BC), being characterized by a high metastatic rate and a poor prognosis due to its tendency to relapse and the lack of targeted therapies. Inhibition of NAD+ synthesis, mostly by targeting Nicotinamide Phosphoribosyltransferase in the salvage pathway, showed promise as anticancer therapy, as NAD+ is implicated in tumor growth, energy production, motion and invasiveness, immune response, and multi-drug resistance. We investigated the role of Nicotinate Phosphoribosyltransferase (NAPRT), the rate-limiting enzyme for NAD+ synthesis starting from nicotinate, in TNBC proliferation, invasiveness, also through the use of a 3D culture model, and in Ca2+ homeostasis. Our results demonstrated that NAPRT promotes tumor aggression and determines the concomitant increase of the cytosolic Ca2+ levels and the decrease in state of replenishment of Thapsigargin-sensitive ER Ca2+ stores. Also, NAPRT overexpression increased the levels of the Ca2+-mobilizing messengers cyclic ADPR and Nicotinic Acid Adenine Dinucleotide Phosphate (measurements with a newly developed assay) and the pharmacological CD38 or SARM1 inhibition reverted the NAPRT-induced changes in Ca2+ homeostasis. Therefore, targeting NAPRT may represent a promising strategy to slow down TNBC growth and progression.